1. Field of the Invention
This invention relates in general to injectors for dispensing fluids in fine sprays, and more particularly relates to fuel injectors for dispensing liquid fuel in fine sprays for ignition in gas turbine engines.
2. Description of the Prior Art
The art of producing sprays of liquid is extensive. Many injectors have a nozzle with a swirl chamber. One or more angled inlet slots direct the fluid to be sprayed into the swirl chamber. The inlet slots cause the fluid to create a vortex in the swirl chamber adjacent to a spray orifice. The fluid then exits through the spray orifice in a conical spray. Patents showing such injectors include U.S. Pat. Nos. 4,613,079 and 4,134,606.
It is believed it is much easier to design and manufacture relatively large nozzles for producing relatively large droplet sprays than to design and manufacture relatively small nozzles to produce relatively fine droplet sprays. This is especially true in the context of manufacturing the inlet slots, swirl chambers, and spray orifices in small nozzles.
In the combustion of fuels, for example, a nozzle that provides a spray of fine droplets improves the efficiency of combustion and reduces the production of undesirable air pollutants. In some applications, it is desirable to have very low Flow Numbers and Flow Numbers that vary from location to location. The “Flow Number” relates the rate of fluid flow output to the applied inlet pressure. Flow Numbers that are less than 1.0 lb/hr.psi0.5, and even as small as 0.1 lb/hr.psi0.5, are desirable in some applications. This corresponds to swirl chambers less than 1.905 mm (0.075 inches); and exit orifices of less than 0.3048 mm (0.012 inches) diameter.
It is believed that for many years it was only possible to manufacture many of the openings and surfaces of small nozzles to create such low Flow Numbers by using relatively low volume machine tool and hand tool operations in connection with high magnification and examination techniques. This was a labor-intensive process with a high rejection or scrap rate.
One technique which has overcome this problem and produces spray nozzles having Flow Numbers as low as 0.1 lb/hr.psi0.5 is described and illustrated in U.S. Pat. No. 5,435,884. In this patent, which is owned by the assignee of the present application, a nozzle having a small swirl chamber, exit orifice and feed slots is provided that produces a fine droplet spray. The swirl chamber, exit orifice and feed slots are formed by chemical etching the surfaces of a thin metal plate. The etching produces a nozzle with very streamlined geometries thereby resulting in significant reductions in pressure losses and enhanced spray performance. The chemical etching process is easily repeatable and highly accurate, and can produce multiple nozzles on a single plate for individual or simultaneous use.
The nozzle shown and described in the '884 patent has many advantages over the prior art, mechanically-formed nozzles, and has received acceptance in the marketplace. The nozzle has design features that allow it to be integrated into an affordable multi-point fuel injection scheme. Nevertheless, the power generation industry is faced with increasingly stringent emissions requirements for ozone precursors, such as nitrogen oxides (NOX) and carbon monoxide (CO). To achieve lower pollutant emissions, gas turbine manufacturers have adopted lean premixed (LP) combustion as a standard technique. LP combustion achieves low levels of pollutant emissions without additional hardware for steam injection or selective catalytic reduction. By premixing the fuel and air, localized regions of near stoichiometric fuel-air mixtures are avoided and a subsequent reduction in thermal NOX can be realized.
To achieve lower levels of NOX emissions, homogeneous fuel-air mixture distributions are necessary. While the nozzle shown in the '884 patent is appropriate for many applications, it does not have an integral air swirler allowing the introduction of the fuel spray into an air flow.
While many of the known air swirlers could be used with the nozzle shown in the '884 patent, such known air swirlers are typically produced by machining or otherwise mechanically-forming the air passages, which would substantially increase the weight and size of the nozzle in the '884 patent. Such swirlers would also be difficult to manufacture in small detail because of the aforementioned problems associated with conventionally machining small parts.
It is therefore believed there is a demand for an injector with a nozzle that provides a spray of fine droplets of a first fluid, and includes integral, compact and lightweight structure that allows the introduction of a second fluid into or in conjunction with the first fluid. It is further believed that there is a demand, particularly for gas turbine applications, for an injector that has a nozzle with a low Flow Number and has an integral, compact and light-weight air swirler to reduce NOX and CO emissions, improve spray patternization, and provide a spray that is well dispersed for efficient combustion.